What research department?
You would imagine that the research department has seen Japanese shoulders. Since this was made in 1941 Japanese shoulders were shown in news reels and the daily papers.
With that being said why we're the Japanese soldiers in this film dressed with German helmets? Might I add new shiny ones at that!
Also, the obvious plot hole is that no one bothered to tell the officers who were summarily taken off the street, their original detachment where they were. Had they Gable's character would have been exposed.
Dramatic license I suppose but audiences at that time would pick up on this fore many were in active service. After all the war was on.